Vacuum Isp of jet engines

[progressiveMonkey]

[Mod Edit: Moved from a year-old thread in General Discussion]

Hi guys,

That's an old thread but I hope still relevant. I do have a question about the quoted Isp of jet engines in vacuum. arq just quoted an Isp of 1000s for the basic jet engine in vacuum, 1200s for the turbojet. What do those values mean? Jet engines cannot operate in vacuum due to the lack of intake air, so IMHO these values should be N/A.

Are these theoretic values, if you fed them liquid oxidiser?

Generally speaking, when building rockets with jet propulsion for the first stages, I feel there is less information to know how efficient what you're doing is. Or maybe it's just that rocket engines are simpler...

Edited August 2, 2014 by technicalfool

[Claw]

The ISP for jet engines "in vacuum" is used by KSP for calculating the ISP curve. As you get higher in altitude and the air thins out, the ISP increases. You are also correct though, in that they will eventually run out of intake air and stop working.

Cheers,

~Claw

[Michaelo90]

Actually, it IS possible to run jets in vacuum, if you bring some intake air with you.

[FonVegen]

Actually, it IS possible to run jets in vacuum, if you bring some intake air with you.

But in order to have significant amounts of Dv, you would need hundreds of full air intakes; so basically you would have to build something which is entirely covered in intakes, close them all while they're full of air and fly that thing into space using rocket power, then fly whereever you want using jet engines...

That sounds like a task for Danny. Or Xacktar.

[GoSlash27]

PM,

Isp is basically a function of the mass of your exhaust and the velocity you're expelling it at. The exhaust is accelerated through a de Laval (or "con-di") nozzle to get supersonic exhaust velocity. Having air pressure behind the exhaust interferes with the process, lowering efficiency. So the vacuum is the theoretical "perfect" Isp while the atmospheric is the Isp at sea level. The reason it's listed is what Claw said.

Let's say you sent up a test rig into orbit with a jet engine, intake air, and fuel. You note it's mass and velocity. Now burn prograde to establish the highest apoapsis you can. Note the mass of fuel consumed.

When you return to periapsis, note the new velocity.

You now have all the info you need to establish the vacuum Isp of a jet engine. You have Mw/Md and DV, so it's just a matter of juggling the rocket equation a little and plugging it in.

DV=9.81Isp*ln(Mw/Md) becomes

Isp=DV/[9.81*ln(Mw/Md))]

Best,

-Slashy

[Claw]

Actually, it IS possible to run jets in vacuum, if you bring some intake air with you.

Yes, this is technically true. If you want to go that far, there is also a mod that lets you take "bottled air" with.

~Claw

[JokingHinge]

Take note that if you use air in space, the thrust decreases massively.

[Michaelo90]

Take note that if you use air in space, the thrust decreases massively.

Depends on where you are, since the thrust depends on your speed. In LKO it would have very low thrust, but in Mun orbit it should have more thrust than on the runway Don't ask me why you'd want to do that though

[Claw]

Don't ask me why you'd want to do that though

You're probably thinking Thrust to Weight Ratio. Gravity on Mun is lower so the TWR is higher when compared to Kerbin.

[Laie]

You're probably thinking Thrust to Weight Ratio. Gravity on Mun is lower so the TWR is higher when compared to Kerbin.

Thrust-by-surface_speed? About 2200m/s in LKO, negligible thrust even if you brought air. But around the MUN, you only make like 500m/s. That air should really get you somewhere.

[Claw]

Ahh, right. I forgot about the fact that the jet engine thrust curve is also adjusted for speed. So yeah, I see what you're saying now.

[progressiveMonkey]

Thanks guys for all the answers, it makes things clearer. Is there a place however where curves of thrust vs. speed and thrust vs. altitudes can be seen for the different jet engines and air intakes? I'm still finding myself heavily guessing when building a rocket with jet assistance, and would like to do a cleaner job!

[Michaelo90]

Javascript is disabled. View full album

That should tell you everything you want to know and more about jets

[Dispatcher]

A simplistic approach to jet engine iSP values is to use the average between surface and vacuum values as a practical value.